Function substitute

pub fn substitute<'a, 'b>(
    s: &'a str,
    vars: &'b HashMap<&'b str, &'b str>,
    used_vars: &mut HashSet<String>,
    max_recursion_depth: u8,
) -> Option<String>

Given a map of key/value pairs, perform variable substitution on a given input string. max_recursion_depth controls the maximum allowed recursion depth if variables refer to other strings themselves containing variable references. A small number but reasonable is recommended by default, e.g. 16. If None is returned, substitution was impossible, either because a referenced variable did not exist, or recursion depth was exceeded.